1. Field of the Invention
The present invention relates to a wireless communication system, and more particularly, to a method for handling Random Access Response reception in a user equipment and an E-UTRAN and related devices.
2. Description of the Prior Art
With the arrival of the information age, the requirements for mobile voice and data communications, and various other mobile services are increasing daily. The communication systems of prior art are facing bottlenecks of channel usage overflow and inefficient transmission speeds. Therefore, fourth generation mobile communication has been developed to make better use of the frequency spectrum and higher-speed transmission service. Fourth generation mobile communication has various versions, of which presently the long term evolution (LTE) system provided with the Third Generation Project Partnership (3GPP) standard is the one most used worldwide due to considerations of system compatibility and manufacturer support.
Please refer to FIG. 1. FIG. 1 is a diagram showing the architecture of the radio interface protocol of a LTE system according to the prior art. As shown in FIG. 1, the radio interface protocol of the LTE system includes three layers: the Physical Layer (L1), the Data Link Layer (L2), and the Network Layer (L3), wherein a control plane of L3 is a Radio Resource Control (RRC) layer, and L2 is further divided into a Packet Data Convergence Protocol (PDCP) layer, a Radio Link Control (RLC) layer, and a Medium Access Control (MAC) layer.
FIG. 2 is a diagram showing a Random Access (RA) procedure of the LTE system according to the prior art. As can be seen from FIG. 2, when a user equipment (UE) 210 initials an RA procedure, an RA preamble is transmitted from the UE 210 to the Evolved UMTS Terrestrial Radio Access Network (E-UTRAN) 220. The E-UTRAN 220 needs to transmit an RA response (RAR) corresponding to the RA preamble to the UE 210. After that, the UE 210 transmits a media access control protocol data unit (MAC PDU) to the E-UTRAN 220, wherein the MAC PDU usually consists of a MAC header and one or more MAC Control Elements (CE). A MAC RAR usually consists of three fields: TA/UL Grant/Temporary C-RNTI (as shown in FIG. 3).
However, the UL Grant field included in the MAC RAR is not defined clearly, and it is not clear how to handle the UL grant field in some scenarios. For example, how to handle the UL grant field in the MAC RAR is not specified in the 3GPP specification, especially when the UE has no data for transmission. In addition, the 3GPP specification defines that the eNB of the E-UTRAN should not provide a grant field smaller than 80 bits in the RA response when an uplink transmission is required. But this limitation is not suitable in some scenarios. Hence, a mechanism for handling the UL grant field in the MAC RAR needs to be improved.